Well, now that the weather is starting to look neat around here,
I am once again trying to put my weather station together.
I have a Motorola MPX5100A sensor (Temp compensated, calibrated,
conditioned sensor) that outputs 0.5- 4.5V for 0-15PSI. Since
atmospheric pressure is around 14.6 PSI, the output voltage from
this sensor will only sweep a small range for a 29"- 31" pressure
change (4.2 - 4.7V)
Can I use a differential serial ADC like the National ADC0831
and feed it the output of the pressure sensor and hold the V-
input at 4.2V? Or is that too tight of a range (4.2 - 4.7V) for
a full 8 bit count?
Thanks
Dave

David Schmidt wrote:
>
> Well, now that the weather is starting to look neat around here,
> I am once again trying to put my weather station together.
> I have a Motorola MPX5100A sensor (Temp compensated, calibrated,
> conditioned sensor) that outputs 0.5- 4.5V for 0-15PSI. Since
> atmospheric pressure is around 14.6 PSI, the output voltage from
> this sensor will only sweep a small range for a 29"- 31" pressure
> change (4.2 - 4.7V)
> Can I use a differential serial ADC like the National ADC0831
> and feed it the output of the pressure sensor and hold the V-
> input at 4.2V? Or is that too tight of a range (4.2 - 4.7V) for
> a full 8 bit count?
> Thanks
> Dave

I don't know if your sensor cames with an anti aliasing filter and
sample and hold circuit, you will have to see the data sheets in order
to know if you have to design one (low pass filter with cutoff about
10Hz for pressure proposes). As for the ADC a good method is subtracting
4.2 V to your signal and then amplifie it by 10.

At 01:23 PM 11/5/96 -0800, David Schmidt wrote:
>Well, now that the weather is starting to look neat around here,
>I am once again trying to put my weather station together.
> I have a Motorola MPX5100A sensor (Temp compensated, calibrated,
>conditioned sensor) that outputs 0.5- 4.5V for 0-15PSI. Since
>atmospheric pressure is around 14.6 PSI, the output voltage from
>this sensor will only sweep a small range for a 29"- 31" pressure
>change (4.2 - 4.7V)
> Can I use a differential serial ADC like the National ADC0831
>and feed it the output of the pressure sensor and hold the V-
>input at 4.2V? Or is that too tight of a range (4.2 - 4.7V) for
>a full 8 bit count?

I built a barometer using a 16C84A and the Motorola MPX4115A (which has
a pressure range of 15-115kPa), and have been very pleased with the
results. However, there are some things you might want to consider:

- The Motorola MPX family of pressure sensors are not guaranteed to
give valid results outside of their rated pressure range. The
high end of the MPX5100A range is 100kPa, which is slightly below
the average sea level pressure (101.32kPa). I haven't seen any
problems when that close to the rated range, but you might consider
the MPX4115A, which ranges up to 115kPa (33.96" of mercury at sea
level).

- For your desired range (29"-31"), an 8-bit ADC will give you 255
steps for a 2" range -- not particularly fine resolution. I used
the Linear Technology LTC1286, which provides 12 bits (4095 steps).
The LTC1286 also allows the use of an external reference voltage.
My project uses a reference voltage of 2.5V, which results in about
3 bits of resolution per hundredth of an inch of mercury.

- I don't have the ADC8031 datasheet in front of me, but I seem to
remember that the linearity error increases significantly as the
reference voltage (V+ minus V-) drops below about 2V. I suspect
that you'd have real problems with a Vref of 0.5V (4.7-4.2). The
LTC1286 is rated for reference voltages down to 1.5V.

- Don't forget that barometric pressure decreases pretty quickly
with increasing altitude. What you'll be measuring is commonly
known as "station pressure", and may be considerably less than
the "(adjusted for) sea level" pressure that we typically see in
weather reports or hear on weather radio. The conversion to
"sea level" pressure is a whole different subject, and non-trivial
on a PIC; e-mail me if you want the gory details.

- It's good to use a decoupling filter (capacitor & resistor) between
the pressure sensor output and the ADC input. See the Motorola
datasheet for details.

To summarize: I got excellent results using a PIC with a Motorola MPX4115A
pressure sensor and a Linear Technology LTC1286 ADC. But getting the
local "station" pressure is just half the battle (or fun...). Having said
that, I'd encourage you to keep going on your project. It doesn't cost
much, and results in a very useful weather instrument.